Your computer isnt too bad at the moment, but your cpu is really showing the age. If you want to know when to upgrade...I'd recomend to get a new computer when Longhorn comes out so your computer will be set to run the new windows os very nicely indeed. Your graphics card right now is still good but its also showing its age. You should upgrade everything in a year or two rather than upgrade some parts in six months, then upgrade other parts in a years time.
Core 2 Duo E6400, DFI Infinity 975X/G, 2x 512mb DDR2 667mhz, Albatron 7900gt, WD 200gb SATA, Samsung DVD-RW, Silverstone ST-50EF 500w PSU.